In an on-going outbreak in rural Brazil though there had been no human-to-human transmission through Aedes aegypti mosquitoes, the infection had spread via non-human forest-dwelling primates, write Infectious Disease Fellow Catharine I Paules, MD, and NIAID Director Anthony S Fauci, MD, in an article published online yesterday in the New England Journal of Medicine.

But an increasing number of people have also been infected, making it Brazil's worst yellow fever outbreak among humans in decades.

Dr. Fauci and Dr. Paules also warned that USA territories like Puerto Rico - where more than 37,000 people have contracted Zika since January 2015 - could be vulnerable to outbreaks of yellow fever. Further spread would be likely in the age of global travel. But for yellow fever to gain a foothold in this country, many of those mosquitoes would need to feast on the blood of people who had been infected elsewhere and traveled here.

But, they added, "it is possible that travel-related cases of yellow fever could occur, with brief periods of local transmission in warmer regions such as the Gulf Coast states, where A. aegypti mosquitoes are prevalent".

USA territories - including Puerto Rico, where the Zika outbreak reached previous year - could also be at risk.

A viral disease found in tropical Africa and the Americas, yellow fever mainly affects humans and monkeys. It claimed hundreds of thousands of lives worldwide in the 18th and 19th centuries.

There is a vaccination against yellow fever, according to the World Health Organization, which provides lifetime protection against the disease. However, Fauci and Paules note the vaccine is not routinely used in Brazil's urban populations.

Top infectious disease experts are warning about a rapidly spreading outbreak of deadly yellow fever in Brazil that could hit parts of the United States. Now, though, the health ministry is rushing to vaccinate people, sending almost 15 million extra doses to the newly affected areas. As vaccines ran low, health officials had to administer one-fifth of the standard dose to extend the supply, Paules and Fauci noted.

It's a perilous moment, made more so by the fact that, while an effective vaccine against yellow fever has been around since 1937, worldwide stockpiles are all but depleted.

While the likelihood of yellow fever outbreaks in the U.S. is low, "travel-related cases of yellow fever could occur, with brief periods of local transmission in warmer regions such as the Gulf Coast states", wrote Dr. Anthony Fauci, director of National Institute of Allergy and Infectious Diseases and colleague Dr. Catharine Paules.